Drilling of terrestrial bores



Patented Aug. 18, 1931 UNITED STATES PATENT OFFICE ALBERT GEYER LOOMIS, HENRY ALFRED AMBROSE, AND JASPER SUMNER BROWN, OF

PITTSBURGH, PENNSYLVANIA, ASSIGNORS 'IO GULF PRODUCTION COMPANY, A 003- PORATION OF TEXAS DBILLING- OF TERRES'I'RIAL BORES No Drawing.

Our invention relates to the drilling of terrestrial bores: itfinds practical application in the drilling of oil and gas wells. Such wells must often be sunk to depths of a thousand feet and more, and through sedimentary rock of various kinds. As drilling progresses, the cuttings broken free by the drill must be removed from the bore; and, in order to effect the removal of the cuttings as w they are broken away, to lubricate the drill,

and to seal off gas pockets, mud-fluid is introduced into the drill hole. In rotary drilling, the mud-fluid is pumped down the drill stem to the drill at the working face in the 715 bottom of the bore; thence the stream passes across the working face and makes escape upward through the bore. The mud-fluid is itself of such high specific gravity that it carries with it in its rising stream the freed m cuttings. At the mouth of the Well the transported cuttings are separated from the mud-fluid, which is ordinarily returned, in-

practically continuous circulation through the well.

When such a well is sunk through certain formations (and we specify, by way of example, the Jackson shale formation of the TeXasoil-fields), a very serious difficulty is often encountered. After the bore has been sunk, the tools are removed and a casing of pipe is lowered into the bore. 1n the formations to which we here refer a phenomenon called heaving occurs. The term heaving connotes the caving of the walls of the hole: after the tools have been removed the bore diminishes in sizebecomes chokedand this choking of the bore may be so pronounced-it may proceed to such extentthat the necessity arises of drilling the bore out again. It even may happen, and indeed generally does happen, that, in consequence of heaving, the well has to be abandoned. The phenomenon of heaving, then,,is very costly in its effects.

The'mud-fiuid which is used in these welldrilling operations is essentially a. mixture of clay with water". A typical mud-fluid may weigh from 8 to 12 pounds to the gallon. Its specific gravity may beincreased to a still 50 higher figure by additions of other, heavier 'Application filed April' 22,

1930. Serial in. 446,409.

material of insoluble nature and,-necessarily, in finely divided condition: barium sulfate for example; or hematite. The mass is of fluidity suflicient for the purposes described. Mud fluids having such composition and characteristics we recognize to be normal mud-fluids.

Examination of shales in which heaving occurs reveals the presence of such soluble salts as the chlorides, the sulfates, and the nitrites of magnesium, calcium, and sodium. Minute crystals of one or more of these substances may be found, distributed through the body of the shale. We have perceived-- and the perception is fundamental to our in- M vention-that water from the mud-fluid which fills the bore of the Well will penetrate the bore-Walls and will by absorption, osmosis, or capillary condensation, or a combination of these phenomena, enter the body of such a shale. The water so entering the shale will dissolve those native crystals of soluble salt which it finds, and thus within the body of shale small pockets of saturated solutions of the salts will be formed. A condition then exists of relatively pure water (that is to say, having a relatively small burden of dissolved matter) in the mud-fluid in the bore and of saturated salt solution within the body of the shale, the two bodies being separated by walls of shale through which, by the nature of the material, small capillary spaces extend, Osmosis will occur-a further rapid movement of water from the mud-fluid to the pockets of solution Within the body of the shale. The osmotic pressure under which this movement occurs is very great: it is a matter of calculation to demonstrate that the osmotic pressure for a saturated solution of sodium chloride, for example, is more than three hundred atmospheres. The rapid increase of water entering the substance of the shale under such pressure and at a multitude of points has its aggregate effect in the phenomenon known as heaving. While it is true that osmosis is an important cause of heaving, there are doubtless other incidental and contributing causative factors. There is a swelling of theshale itself as it absorbs watera swelling which 100 would occur, even though there were no. soluble salts within it; and there is the dis-' persion of the colloids of the shale. These activities and perhaps others are in progressg tending to the same endthat movement the borewalls to which the term heaving so rapid as to freeze the drill, so that it can not be pulled out of the bore hole.

Having discovered the causes of heaving, we have further perceived that, by causing the water present in the mud-fluid to take material into solution; or, in case it already carries material in solution, by increasing its burden of dissolved material; its activity as an agent tending to penetrate the shale formation which constitutes the b0re-wall may be diminished. Specifically, the increase in concentration of dissolved material of the solution may be carried to such extent that the Water loses entirely its tendency to penetrate the shale; and then the heavin is not merely diminished in amount; it is, indeed, completely eliminated from the operation.

We do not mean to be understood to say that, in the ordinary run of drilling operations, the mud-fluids employed do not carry mineral matter in solution. They do. Probably some content of dissolved mineral is present in every mud-fluid. But we do mean to be understood to say that we have discovered a relationship between such condition of solution and the phenomenon of heavng,'and following upon that discovery we have reached the practical result that, by purposefully building-up the concentration of dissolved material in solution in the water of the mud-fluid, we may correct the fault of heaving, even to the point of its complete prevention. A more exact scientific state ment may be made by saying that, by solution of certain substances (such as calcium chloride) in the mud-fluid, the activity of the water in the mud may be decreased to a point below that of the activity ofthe Water in a saturated solution of the salts within the shale. The result is the elimination of osmosis, by the removal of the driving force required to draw water into the shale.

While the principle holds that any increased concentration of dissolved material within the mud-fluid has the effect of reducing the degree to which heaving will progress, some soluble substances are more effective than others. Generally speaking, the more readily soluble the substance chosen is, in terms of molal concentration, the more effective it is to prevent heaving. And we have found that if we efiect within the mud-fluid a saturated solution of a substance equally soluble with or more soluble than the native salts within the body of the shale, heaving will to all intents and purposes be wholly prevented.

These shales contain, as has been said, such salts as the chlorides, sulfates, and nitrites of magnesium, calcium, and sodium; and of these sodium chloride, magnesium chloride, and calcium sulfate Wlll commonly be found to preponderata In practicing our invention, it may be convenlent to use in mixing the mud-fluid the water which comes from the formation which is being drilled and which will contain in solution these same salts. To the mixture we add an excess of the salts already present; or, preferabl of a more soluble salt, and (most availablb of all, and under ordinar conditions most serviceable), preferablyo calcium chloride. We preferably add such salt insuflicient quantity to insuresolution to the pointof saturation. For all ordinary operations in the oil fields, calcium chloride, because of its high solubility and because of its cheapness' as well, is the best material known to us. But it will be understood that the invention centers in the condition of solution rather than in the particular material dissolved.

Chloride of calcium is ordinarily best; chloride of sodium may be adequate, even in case the soluble mineral in the formation is chloride of sodium. Other available solutes are chloride of magnesium, and the sulfates and the nitrites of calcium and magnesium. Even soluble organic substances are available. As a matter of performance, sugar is adequate, although it is economically unsuited to the purpose. Even though the dis--- solved substance be less soluble than the native salts within the shale, it will be effective to diminish heaving; but the best results are attained by choosing a substance which is more soluble.

As we have said, the invention is permissive of, use in such manner as more or less completely to prevent heaving. Any purposeful addition of soluble substance to the mud-fluid, with the consequent diminution of heaving is a practice of the invention.

The procedure in the drilling of a well will conveniently be to employ -ml1d-flllld prepared in the usual manner, until that formation is reached within which heavin likely to occur. Thereupon the muduid will be modified by the addition to it of soluble material, according to the foregoing explanation, and with :the so modified mudfluid the operation of drilling will proceed.)

We have made tests and have found that the viscosity of the mud-fluid is not impaired by the addition to it of water-soluble salts,

llO

and specifically of calcium chloride. Nor isthe ability of the water impaired to carry the particles of clay in suspension. Indeed, this ability to carry the clay in suspension may be increased by adding to the mud-fluid such acheap material as naphthenic acidsoap.

The practice of our invention introduces no health hazard, so far as concerns the drillers; and the corrosive effect of the material is not practically difl'erent nor greater than that of the brines ordinarily encountered in such drilling operations.

This invention has been found successful in preventing heaving in drilling a well through the Sylvan shale formation in Oklahoma, a formation similar to the Jackson formation of Texas in so far as heaving is concerned. 'In this test the bore was made with cable tools.

We claim as our invention:

1. In thes'inkin of a terrestrial bore with the aid of a mud uid to and through a formation susceptible to heaving due to the presence of water-soluble salts therein, the method hereindescribed which consists in conducting the drilling operation with the aid of a normal mud fluid until the formation susceptible to heaving is reached, then modifyin the mud fluid by increasinv its burden 0 dissolved matter, and proceeding with the drilling operation with the aid of the mud fluid so modified.

2. In the sinking of a terrestrial bore with the aid of a mud fluid toand through a shale formation in which water-soluble salts are found, the method herein described which consists in conducting the drilling operation with the aid of a normal mud fluid until such shale formation is reached, then modifying the mud fluid by adding to it a substance as soluble at least as the salts present in the shale formation, and proceeding with the drilling operation with the aid of the mud fluid so modified.

3. In the sinking of a terrestrial bore with the aid of a mud fluid through a formation susceptible to heaving due to the presence of water-soluble salts therein, the method herein described which consists in providing a mud fluid having a burden of dissolved matter of a magnitude substantially preventive of penetration into such formation and proceeding with the drilling operation with the aid of such mud fluid.

In testimony whereof we have hereunto set our. hands. ALBERT. GEYER LOOMIS. HENRY ALFRED AMBROSE. JASPER- SUMNER BROWN. 

